Heat-softenable toners are widely used in imaging methods such as electrostatography, wherein electrically charged toner is deposited imagewise on a dielectric or photoconductive element bearing an electrostatic latent image. Most often in such methods, the toner is then transferred to a surface of another substrate, such as, e.g., a receiver sheet comprising paper or a transparent film, where it is then fixed in place to yield the final desired toner image.
When heat-softenable toners, comprising, e.g., thermoplastic polymeric binders, are employed, the usual method of fixing the toner in place involves applying heat to the toner once it is on the receiver sheet surface to soften it and then allowing or causing the toner to cool.
One such well-known fusing method comprises passing the toner-bearing receiver sheet through the nip formed by a pair of opposing rollers, at least one of which (usually referred to as a fuser roller) is heated and contacts the toner-bearing surface of the receiver sheet in order to heat and soften the toner. The other roller (usually referred to as a pressure roller) serves to press the receiver sheet into contact with the fuser roller.
The fuser roller usually comprises a rigid core covered with a resilient material, which will be referred to herein as a "base cushion layer." The resilient base cushion layer and the amount of pressure exerted by the pressure roller serve to establish the area of contact of the fuser roller with the toner-bearing surface of the receiver sheet as it passes through the nip of the pair of rollers. The size of this area of contact helps to establish the length of time that any given portion of the toner image will be in contact with and heated by the fuser roller. The degree of hardness (often referred to as "storage modulus") and stability thereof, of the base cushion layer are important factors in establishing and maintaining the desired area of contact.
Also, often the pressure roller and fuser roller have a regular cylindrical shape, but it has been found in the prior art to be advantageous in some cases to change the shape of the pressure roller in order to vary the amount of pressure exerted by the pressure roller against the receiver sheet and fuser roller. This variance of pressure, in the form of a gradient of pressure that changes along the direction through the nip that is parallel to the axes of the rollers, can be established, for example, by continuously varying the overall diameter of the pressure roller along the direction of its axis such that the diameter is smallest at the midpoint of the axis and largest at the ends of the axis, in order to give the pressure roller a sort of "bow tie" or "hourglass" shape. This will cause the pair of rollers to exert more pressure on the receiver sheet in the nip in the areas near the ends of the rollers than in the area about the midpoint of the rollers. It is believed that this gradient of pressure helps to prevent wrinkles and cockle in the receiver sheet as it passes through the nip.
However, if, over time of use, the fuser roller begins to permanently deform to conform to the shape of the pressure roller, the gradient of pressure will be reduced or lost, along with its attendant benefits. It has been found that permanent deformation (alternatively referred to as "creep") of the base cushion layer of the fuser roller is the greatest contributor to this problem.
In the past, it was thought that various materials' suitability for use in fuser roller base cushion layers in terms of their stability during use--i.e., their ability to resist degradation (as evidenced by weight loss), creep, and changes in hardness, during use in fuser rollers--could be determined by subjecting samples of the materials to conditions of continuous high temperature and continuous high stress (i.e., pressure), and then measuring the resultant changes in weight, shape (e.g., length), and hardness (e.g., storage modulus). However, J. J. Fitzgerald et al, "The Effect of Cyclic Stress on the Physical Properties of a Poly(Dimethylsiloxane) Elastomer", Polymer Engineering and Science, Vol. 32, No. 18, (September 1992), pp. 1350-1357, indicates that such testing does not accurately portray the stability the materials will exhibit during actual use in fuser roller base cushion layers and that dynamic testing, with cycles of loading and unloading is necessary. The publication cites other reports showing the same kind of results in studies of other elastomers.
Fuser roller materials can be conveniently tested under conditions of cylic stress using a Mechanical Energy Resolver.TM. (also referred to herein as an "MER") commercially available from Instrumentors, Inc. This device applies heat continuously to maintain the samples at a constant elevated temperature. The device also applies stress to the samples in the form of a compressive force, but does so in a manner such that the amount of compressive force applied varies cyclicly (i.e., sinusoidally). The results of such testing consistently correlate with, and therefore reliably predict, the degree of stability a material will exhibit in the base cushion layer of a fuser roller during actual use.
Another consideration for fuser rollers is the materials that will contact the rollers during use. In a typical electrophotographic process fusing subsystem there are multiple sets of rollers. In order to prevent toner build-up on the rollers, image degradation, hot offset, and toner contamination problems which may decrease fuser roller life, release oil is often applied to the fusing roller.
The release oil is typically poly(dimethylsiloxane) oil (also referred to herein as "PDMS oil"), which is selected for its ability to withstand the almost continuous high temperatures (.about.200.degree. C.) of the electrophotographic fusing process. While PDMS oil does an excellent job in its role as release agent, its compatibility with PDMS-based roller materials results in swelling of the rollers. This swelling cannot be easily compensated for, since it is generally non-uniform. Paper passing over the rollers can wick away some of the release oil within the paper path, resulting in a differential availability of the release oil to roller areas within and outside the paper path. This causes differential swell of the roller inside and outside the paper path so that a "step pattern" is formed in the roller. This can cause problems when different size papers are used and can lead to increased wear and decreased roller life.
One type of material that has been widely employed in the past to form a resilient base cushion layer for fuser rollers is condensation-crosslinked poly(dimethylsiloxane) elastomer. "Poly(dimethyl-siloxane)" will sometimes be alternatively referred to herein as "PDMS". The prior art has also taught or suggested that various fillers comprising inorganic particulate materials can be included in such PDMS base cushion layers to improve their mechanical strength and/or thermal conductivity. Higher thermal conductivity is advantageous when the fuser roller is heated by an internal heater, so that the heat can be efficiently and quickly transmitted toward the outer surface of the fuser roller and toward the toner on the receiver sheet that it is intended to contact and fuse. Higher thermal conductivity is not so important when the roller is intended to be heated by an external heat source. Disclosure of such filled condensation-cured PDMS elastomers for fuser rollers can be found, for example, in U.S. Pat. Nos., 4,373,239; 4,430,406; and 4,518,655.
One specific example of a condensation-crosslinked PDMS elastomer, which contains 32-37 volume percent (vol %) aluminum oxide filler and about 2-6 volume percent iron oxide filler, and which has been widely used and taught to be useful in fuser rollers, is sold under the tradename Stycast.TM. 4952 by Grace Specialty Polymers, W. R. Grace & Co. However, it has been found that fuser rollers containing Stycast.TM. 4952 cushion layers exhibit serious stability problems over time of use, i.e., significant degradation, creep, and changes in hardness, that greatly reduce their useful life. MER test results correlate with and predict the instability exhibited during actual use. Nevertheless, materials such as Stycast.TM. 4952 initially provide very suitable resilience, hardness, and thermal conductivity for fuser roller cushion layers.
Some condensation-crosslinked PDMS elastomers that show less change in hardness and creep than Stycast.TM. 4952 or aluminum oxide-filled PDMS are disclosed in U.S. patent application Ser. No. 08/167,584 now U.S. Pat. No. 5,480,724 (tin oxide filler), U.S. Pat. No. 5,292,606 (zinc oxide filler), U.S. Pat. No. 5,269,606 (copper oxide filler), U.S. Pat. No. 5,292,562 (chromium oxide filler), and U.S. Pat. No. 5,336,593 (nickel oxide filler).
U.S. patent application Ser. No. 08/306,066 now U.S. Pat. No. 5,480,725 discloses a tin oxide-filled, addition-cured polysiloxane system containing 0 to less than 20 mol % (mole percent) diphenyl units and the remainder dimethyl units. U.S. patent application Ser. No. 08/363,149 now U.S. Pat. No. 5,587,245 teaches a zinc oxide-filled, addition-cured polysiloxane system containing 0-25% phenyl. U.S. patent application Ser. No. 08/268,136 now U.S. Pat. No. 5,466,533 discloses a zinc oxide-filled, condensation-cured polydimethyl diphenyl siloxane system containing 20-40 wt % (weight percent) zinc oxide and less than 20 mol % polydiphenylsiloxane.
U.S. Pat. No. 4,970,098 by J. Ayala-Esquilin, W. H. Dickstein, J. L. Hedrick, Jr., J. C. Scott, and A. C. Yang discloses a diphenyl-dimethylsiloxane elastomer filled with 40-55 wt % zinc oxide of 100-500 nm (nanometers) particle size, 5-10 wt % graphite of less than 10 .mu.m (micrometers) particle size, and 1-5 wt % ceric dioxide of 0.2-3 .mu.m particle size. The diphenyl content was 20-50 wt % (equivalent to 8.5 to 27 mol %).
U.S. Pat. No. 4,807,341 by P. A. Nielsen and J. A. Pavlisko discloses a diphenyl-dimethylsiloxane elastomer containing 5-15 mol % diphenylsiloxane and 0-5% vinyl-addition crosslinked siloxane units. Aluminum oxide and iron oxide fillers were disclosed.
U.S. Pat. No. 4,074,001 describes fixing rollers for electrophotography which may comprise phenyl-substituted diorganopolysiloxanes filled with calcium carbonate (less than 10 .mu.m particle size), iron oxide (less than 10 .mu.m particle size), and titanium dioxide (less than 10 .mu.m particle size).
U.S. Pat. No. 4,360,566 describes heat fixing rollers for electrophotography that may comprise addition-crosslinked diphenyl-substituted polyorganosiloxanes, filled with substantial amounts (50-250 parts by weight) of siliceous filler.
U.S. Pat. No. 4,454,262 describes silicone rubbers that may contain phenyl radicals and that contain spindle-shaped calcium carbonate filler.
Some of the above references have a variety of shortcomings. Many do not address the issue of improved stability under cyclic stress at elevated temperature optionally accompanied by reduced oil swell. Further, some of the references disclose costly fillers.
It is the objective of the present invention to improve the hardness (storage modulus) stability and minimize the weight loss and the creep (length change) of an addition-cured polysiloxane under conditions of elevated temperature and cyclic stress through the use of copper oxide filler, while also optionally improving the resistance to PDMS oil swell .